WO2022209928A1

WO2022209928A1 - Lithium-ion capacitor

Info

Publication number: WO2022209928A1
Application number: PCT/JP2022/012120
Authority: WO
Inventors: 祥平増田; 健一永光; 菜穂松村
Original assignee: パナソニックＩｐマネジメント株式会社
Priority date: 2021-03-29
Filing date: 2022-03-17
Publication date: 2022-10-06
Also published as: CN117063259A; JPWO2022209928A1; US20240128030A1

Abstract

The disclosed lithium-ion capacitor is configured in a manner such that: the positive electrode thereof is equipped with a positive electrode current collector and a positive electrode mixture layer held by the positive electrode current collector; the positive electrode mixture layer contains a positive electrode active material for reversibly doping an anion; the electrostatic capacity of the positive electrode current collector is no higher than 20μF/cm²; the negative electrode is equipped with a negative electrode current collector and a negative electrode mixture layer held by the negative electrode current collector; the negative electrode mixture layer contains a negative electrode active material for reversibly doping a lithium ion; the electrolyte thereof contains a first lithium salt and a second lithium salt; the first lithium salt is a lithium salt of a fluorine-containing inorganic acid; the second lithium salt is a lithium salt of a fluorine-containing acid imide; and the proportion which the molar concentration of the first lithium salt constitutes in the total molar concentration of the first and second lithium salts in the electrolyte is greater than 0% and no greater than 35%.

Description

lithium ion capacitor

The present invention relates to lithium ion capacitors.

　Lithium-ion capacitors, which combine the storage principle of lithium-ion secondary batteries and electric double-layer capacitors, are attracting attention. A lithium ion capacitor uses a polarizable electrode for the positive electrode and a non-polarizable electrode for the negative electrode. Lithium ion capacitors are expected to combine the high energy density of lithium ion secondary batteries with the high output characteristics of electric double layer capacitors.

Patent Document 1 discloses a non-aqueous lithium-type storage element comprising a laminate film, and a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte hermetically housed in the laminate film, wherein the positive electrode is placed on a positive electrode current collector. has a positive electrode active material layer made of a material containing activated carbon, the negative electrode has a negative electrode active material layer capable of intercalating and deintercalating lithium ions on a negative electrode current collector, and the non-aqueous lithium storage element 1.80≦K/M<4.00, where K (N/mm) is the spring constant in the thickness direction and M (g) is the mass of the non-aqueous lithium-type storage element. We have proposed a non-aqueous lithium-type storage device that It is described that both the positive electrode current collector and the negative electrode current collector are plain foils. Non-aqueous electrolytic solutions include lithium salts such as (LiN(SO ₂ F) ₂ ), LiN(SO ₂ CF ₃ ) ₂ , LiN(SO ₂ C ₂ F ₅ ) ₂ , LiN(SO ₂ CF ₃ ) ( _SO2C2F5 ), LiN( _SO2CF3 ) ₍ _SO2C2F4H ) _, _LiC ₍ _SO2F ) ₃ , _LiC ₍ _SO2CF3 ) ₃ , _LiC ₍ _SO2C2F5 ) ₃ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiPF ₆ , LiBF ₄ and the like can be used alone, and two or more of them can be mixed and used, and since high conductivity can be expressed, LiPF ₆ and/or LiN( _SO2F)2 _.

Japanese Patent Application Laid-Open No. 2019-24040

However, it is common to use metal porous bodies such as perforated foils and etched foils as positive electrode current collectors for lithium ion capacitors. When a plain foil is used as in Patent Document 1, the positive electrode active material layer is easily peeled off, resulting in remarkable deterioration of the positive electrode. In addition, when the electric storage device is used at a high temperature of 80° C. or higher, gas generation may become significant. On the other hand, at a low temperature of 0° C. or less, the initial resistance may increase, and sufficient high output characteristics may not be obtained.

One aspect of the present invention includes a positive electrode, a negative electrode, and a lithium ion conductive electrolyte, wherein the positive electrode includes a positive electrode current collector and a positive electrode mixture layer supported on the positive electrode current collector, The positive electrode mixture layer includes a positive electrode active material reversibly doped with anions, the positive electrode current collector has a capacitance of 20 μF/cm ² or less, and the negative electrode comprises a negative electrode current collector and the negative electrode. a negative electrode mixture layer supported on a current collector, the negative electrode mixture layer including a negative electrode active material that reversibly dopes lithium ions, the electrolyte comprising a first lithium salt and a second a lithium salt, wherein the first lithium salt is a lithium salt of a fluorine-containing inorganic acid, the second lithium salt is a lithium salt of a fluorine-containing acid imide, and the first lithium salt in the electrolyte and the second lithium salt, the ratio of the molar concentration of the first lithium salt to the total molar concentration is more than 0% and 35% or less.

To provide a lithium-ion capacitor that is stable even at high temperatures, whose positive electrode does not easily deteriorate, and whose resistance does not easily increase even at low temperatures.

FIG. 1 is a partially cutaway perspective view of a lithium ion capacitor according to an embodiment of the present disclosure. FIG. 2 is a graph showing the relationship between the DCR at low temperature and the amount of gas generated during float charging at high temperature of a lithium ion capacitor, and the total molar concentration of lithium salts.

A lithium ion capacitor according to the present disclosure includes a positive electrode, a negative electrode and a lithium ion conductive electrolyte. In general, a positive electrode and a negative electrode constitute a capacitor element together with a separator interposed therebetween. The capacitor element is configured, for example, as a columnar wound body by winding a strip-shaped positive electrode and a strip-shaped negative electrode with a separator interposed therebetween. Also, the capacitor element may be configured as a laminate by laminating a plate-like positive electrode and a plate-like negative electrode with a separator interposed therebetween.

Each component of the lithium ion capacitor according to the embodiment of the present invention will be described in detail below.

[Positive electrode]
The positive electrode includes a positive electrode current collector and a positive electrode mixture layer carried on the positive electrode current collector. The positive electrode may be a polarizable electrode, or may be an electrode that has the properties of a polarizable electrode and in which the Faraday reaction also contributes to the capacity.

The positive electrode mixture layer contains a positive electrode active material reversibly doped with anions. When the anion is adsorbed on the positive electrode active material, an electric double layer is formed and the capacity is developed. The doping of the positive electrode active material with the anion includes at least the phenomenon of adsorption of the anion to the positive electrode active material, and is a concept that can also include absorption of the anion by the positive electrode active material, chemical interaction between the positive electrode active material and the anion, and the like. be.

The positive electrode mixture layer contains a positive electrode active material as an essential component, and a conductive material, a binder, and the like as optional components. Examples of conductive materials include carbon black and carbon fiber. Examples of binders include fluorine resins, acrylic resins, rubber materials, and cellulose derivatives. The content of the binder contained in the positive electrode mixture layer may be, for example, 2% by mass or more and 10% by mass or less, or 2% by mass or more and 8% by mass or less.

A carbon material is used as the positive electrode active material. The carbon material is preferably porous, and activated carbon is particularly preferred. Raw materials for activated carbon include, for example, wood, coconut shells, coal, pitch, and phenolic resin. Activated carbon may be activated carbon. The average particle size of the activated carbon is not particularly limited, but is, for example, 20 μm or less, and may be 3 μm or more and 15 μm or less. The average particle diameter is the median diameter in the volume-based particle size distribution, and can be measured, for example, by a laser diffraction particle size distribution analyzer.

It is desirable that the active carbon accounts for 50% by mass or more, further 80% by mass or more, furthermore 95% by mass or more of the positive electrode active material. In addition, it is desirable that the active carbon accounts for 40 mass % or more, further 70 mass % or more, furthermore 90 mass % or more of the positive electrode mixture layer.

The positive electrode mixture layer is formed, for example, by mixing a positive electrode active material, a conductive material, a binder, and the like with a dispersion medium to prepare a positive electrode mixture slurry, and applying the positive electrode mixture slurry to a positive electrode current collector. , formed by drying. A sheet-like metal material is used for the positive electrode current collector.

The thickness of the positive electrode mixture layer is, for example, 10 μm or more and 300 μm or less, may be 30 μm or more and 70 μm or less, or may be 40 μm or more and 60 μm or less per one side of the positive electrode current collector. As the positive electrode mixture layer is thicker, the positive electrode mixture layer is more likely to peel off, but by suppressing deterioration of the positive electrode current collector, a relatively thick positive electrode mixture layer can be formed.

The positive electrode current collector may be a metal foil. Generally, a metal porous body such as a perforated foil or an etched foil is used for the positive electrode current collector of a lithium ion capacitor, but the capacitance of the positive electrode current collector of the lithium ion capacitor according to the present disclosure is 20 μF/cm. It can be ² or less. That is, the positive electrode current collector does not have to be a perforated foil or an etched foil. The positive electrode current collector may be plain foil. A plain foil is a metal foil that does not have a plurality of holes and whose surface is not roughened by etching or the like. The positive electrode current collector may have a capacitance of 4 μF/cm ² or less. The thickness of the positive electrode current collector is, for example, 5 μm or more and 50 μm or less, may be 30 μm or less, or may be 20 μm or less or 15 μm or less. Plain foils have high mechanical strength and may be thinner than typical perforated or etched foils. By using a thin plain foil, it becomes possible to form a thicker positive electrode material mixture layer, which makes it easier to improve the capacity of the lithium ion capacitor.

The electrostatic capacity of X μF/cm ² means that the positive electrode current collector is orthographically projected with the main surface of the positive electrode current collector parallel to the plane of projection, and the capacitance per unit area (1 cm ² ) of the projection view is is the capacitance, and the capacitance X is the total capacitance of the front and back (that is, X/2 per side).

　The capacitance of the metal foil is measured by the following method. First, a test piece of metal foil (that is, positive electrode current collector) is prepared. Although the shape and size of the test piece are not particularly limited, it has a portion to be measured and a lead portion for measuring capacitance. The portion to be measured is the portion that is immersed in the measurement solution. Although the shape and size of the portion to be measured are not particularly limited, a rectangular shape is desirable, and has dimensions of, for example, 10 mm×50 mm. The withdrawn portion is the portion that is not immersed in the measurement solution. The drawn-out portion may be a portion that is cut out integrally with the portion to be measured. The shape and size of the drawer part are arbitrary. As the measurement solution, an aqueous solution of 80 g of ammonium pentaborate dissolved in 1 L of water is used. The capacitance measuring device complies with JIS C 5101-1. The accuracy is ±2% of the measured value, the measurement frequency is 120 Hz ±5%, and the measurement voltage is 0.5 Vrms or less. The measurement tank containing the measurement solution is a glass tall beaker with a capacity of 200 mL or 300 mL conforming to JIS R 3503. Set the temperature of the measurement solution to 30°C ± 1°C, immerse the parts to be measured of a pair of test pieces of the same shape and size in the measurement solution, and apply electrostatic Connect to a capacitance meter to measure the capacitance. The direction of the test piece (portion to be measured) in the measurement solution is arbitrary, but a separation distance of 5 mm±2 mm is provided between the two.

Aluminum or an aluminum alloy is used as the metal material that constitutes the metal foil that is the positive electrode current collector. Aluminum alloys are alloys of aluminum and other metals. Other metals include, but are not limited to, copper, manganese, silicon, magnesium, zinc, and nickel. The content of other elements contained in the aluminum alloy is preferably 10% by mass or less, more preferably 2% by mass or less.

[Electrolytes]
The electrolyte has lithium ion conductivity. The electrolyte includes, for example, a lithium salt and a solvent that dissolves the lithium salt. The anion of the lithium salt reversibly repeats doping and dedoping of the positive electrode. Lithium ions derived from the lithium salt reversibly repeat doping and dedoping of the negative electrode.

The lithium salt includes a first lithium salt and a second lithium salt. The first lithium salt is a lithium salt of a fluorine-containing inorganic acid, and the second lithium salt is a lithium salt of a fluorine-containing acid imide.

The first lithium salt may be, for example, at least one selected from the group consisting of LiPF ₆ , LiBF ₄ , LiSbF ₆ and LiAsF ₆ . Among them, at least one selected from the group consisting of LiPF ₆ and LiBF ₄ is preferable from the viewpoint of DCR reduction. At least one selected from the group consisting of LiPF ₆ and LiBF ₄ may account for 80% by mass or more of the first lithium salt, or 90% by mass or more.

The _second lithium salt is, for example, LiN ₍ _FSO2 ) ₂ , LiN ₍ _CF3SO2 ) ₂ , LiN ₍ _CF3SO2 ) ₍ _C4F9SO2 ) and LiN ₍ _C2F5SO2 ) ₂ It may be at least one selected from the group consisting of Among them, at least one selected from the group consisting of LiN(FSO ₂ ) ₂ and LiN(CF ₃ SO ₂ ) ₂ is preferable, and LiN(FSO ₂ ) ₂ is particularly preferable, from the viewpoint of reducing the amount of gas generated. Hereinafter, LiN(FSO ₂ ) ₂ is also referred to as LiFSI.

By using LiFSI, the rate of increase in low-temperature DCR tends to be significantly reduced. LiFSI is less likely to produce by-products and is thought to contribute to smooth charging and discharging without damaging the surface of the positive electrode active material. LiFSI may account for 80 mass % or more of the second lithium salt, or may account for 90 mass % or more.

Here, the ratio of the molar concentration of the first lithium salt to the total molar concentration of the first lithium salt and the second lithium salt in the electrolyte (hereinafter also referred to as the “proportion of the first lithium salt”. The ratio is the “X” value described later.) is important in providing a lithium ion capacitor that is stable even at high temperatures, the deterioration of the positive electrode does not progress easily, and the resistance does not easily increase even at low temperatures. The ratio of the molar concentration of the first lithium salt to the total molar concentration of the first lithium salt and the second lithium salt is controlled to be greater than 0% and 35% or less. The molar concentration ratio of the first lithium salt may be 10% or more and 30% or less. In this case, the direct current resistance (DCR) at a low temperature of, for example, −30° C. can be maintained low, while gas generation during float charging at 80° C. or higher can be significantly suppressed. Float charging is a charging method that maintains a constant voltage for a long period of time using an external DC power supply. Gas generation is highly dependent on the degree of side reaction between the positive electrode current collector and the lithium salt, and is an indicator of the degree of deterioration of the positive electrode. The greater the gas generation, the greater the deterioration of the positive electrode, and the more likely the positive electrode material mixture layer will peel off.

In the lithium ion capacitor according to the present disclosure, despite using a positive electrode current collector with a very small capacitance, the DCR at low temperatures is kept low, and deterioration of the positive electrode is suppressed at high temperatures of 80 ° C. or higher. This is possible because the ratio of the first lithium salt and the second lithium salt is properly controlled.

Since the positive electrode active material is doped with a large amount of anions during charging, interaction between the positive electrode current collector and the anions is likely to occur unless the proportion of the first lithium salt is properly controlled. At a high temperature of 80° C. or higher, the first lithium salt, which is a lithium salt of a fluorine-containing inorganic acid, undergoes a severe side reaction with aluminum, causing gas generation and deterioration of the positive electrode current collector. In particular, a positive electrode current collector having a capacitance of more than 20 μF/cm ² has a large surface area, so that the side reaction becomes remarkable. In particular, the use of a lithium ion capacitor that continues float charging at a high temperature of 80° C. or higher (further, 85° C. or higher) is used in a considerably harsh environment. If the ratio of the first lithium salt is not properly controlled, it is difficult to use a positive electrode current collector with a capacitance of 20 μF/cm ² or less in such a usage environment.

Furthermore, if the proportion of the first lithium salt is not properly controlled and the proportion of the second lithium salt is too high, the DCR at low temperatures becomes too large. In particular, a positive electrode current collector having a capacitance of 20 μF/cm ² or less has a limited contact area with the positive electrode current collector, so that the DCR increases significantly. In other words, if the ratio of the first lithium salt is not properly controlled, it is difficult to use a positive electrode current collector with a capacitance of 20 μF/cm ² or less also from the viewpoint of DCR.

On the other hand, in the case of a lithium ion capacitor in which the ratio of the first lithium salt is properly controlled, the DCR at low temperatures can be reduced, so a positive electrode current collector with a capacitance of 20 μF/cm ² or less can be used. Along with this, the amount of gas generated at high temperatures is further reduced. Since such a positive electrode current collector has a smooth (that is, plain) surface, it has high strength and can be formed thin, which is advantageous in terms of increasing the energy density of the lithium ion capacitor. Even if the positive electrode mixture layer is formed thick, the positive electrode is less likely to deteriorate, and the separation of the positive electrode mixture layer is less likely to occur.

A desirable correspondence relationship between the ratio of the first lithium salt and the capacitance of the positive electrode current collector can be expressed as follows. When the ratio of the first lithium salt is X% and the capacitance of the positive electrode current collector is Y μF/cm ² , X and Y are expressed by formula (1): Y ≤ X+10 and formula (2): Y ≤ It is desirable to satisfy -0.8X+28. However, 0<X≦35. That is, when the ratio of the first lithium salt is in the range of 0<X≦10, the capacitance Y of the positive electrode current collector satisfies the formula (1), and the ratio of the first lithium salt is in the range of 10≦X≦35. Then, it is desirable that the capacitance Y of the positive electrode current collector satisfies the formula (2). That is, in the range where the ratio of the first lithium salt is small, it is desirable that the electrostatic capacity Y of the positive electrode current collector increases with X, and in the range where the ratio of the first lithium salt is large, the electrostatic capacity of the positive electrode current collector It is desirable that Y be smaller as X is larger. When the capacitance of the positive electrode current collector and the ratio of the first lithium salt are appropriately controlled, the positive electrode can maintain sufficient durability even with a relatively small amount of binder. The content of the binder contained in the positive electrode mixture layer may be 2% by mass or more and 8% by mass or less, or may be 2% by mass or more and 6% by mass or less.

The total molar concentration of the first lithium salt and the second lithium salt in the electrolyte may be 0.7 mol/L or more and 1.3 mol/L or less. However, the lithium salt concentration in the electrolyte is measured using the electrolyte in a discharged state (state of charge (SOC) 0 to 10%). Within this range of total lithium salt molarity, the viscosity of the electrolyte can be kept relatively low while being rich in anions and cations. Therefore, it is advantageous for reducing DCR at low temperatures. Moreover, within the above range, gas is less likely to be generated during float charging at high temperatures, although the electrolyte is rich in anions. This is thought to be because the ions are coordinated with the solvent, making it difficult for the solvent to decompose.

The electrolyte may contain a third salt other than the first lithium salt and the second lithium salt, but 80% by mass or more, further 90% by mass or more in the electrolyte is the first lithium salt and the second lithium salt is preferably occupied by

Examples of the _third salt include LiClO4, _LiAlCl4 , _LiSCN , _LiB10Cl10 , LiCl, LiBr, _LiI , _LiBCl4 , _LiCF3SO3 _, _LiCF3CO2 and the like. These may be used individually by 1 type, or may combine 2 or more types.

Examples of the solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate and butylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate; and aliphatic carboxylic acids such as methyl formate, methyl acetate, methyl propionate and ethyl propionate. acid esters, lactones such as γ-butyrolactone and γ-valerolactone, chain ethers such as 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE) and ethoxymethoxyethane (EME), tetrahydrofuran , cyclic ethers such as 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, acetamide, dimethylformamide, dioxolane, acetonitrile, propionitrile, nitromethane, ethylmonoglyme, trimethoxymethane, sulfolane, methylsulfolane, 1 , 3-propanesultone and the like can be used. These may be used alone or in combination of two or more.

The electrolyte may contain various additives as necessary. For example, an unsaturated carbonate such as vinylene carbonate, vinylethylene carbonate, or divinylethylene carbonate may be added as an additive that forms a lithium ion conductive film on the surface of the negative electrode.

[Negative electrode]
The negative electrode includes a negative electrode current collector and a negative electrode mixture layer carried on the negative electrode current collector. A sheet-like metal material is used for the negative electrode current collector. The thickness of the negative electrode current collector is, for example, 10 μm or more and 300 μm or less. The thickness of the negative electrode mixture layer is, for example, 10 μm or more and 300 μm or less per one side of the negative electrode current collector.

The negative electrode current collector may be a metal foil, a metal porous body, an etched metal, or the like. As metal materials, copper, copper alloys, nickel, stainless steel, and the like can be used.

The negative electrode mixture layer contains a negative electrode active material that reversibly dopes lithium ions. The doping of lithium ions into the negative electrode active material includes at least the absorption phenomenon of lithium ions into the negative electrode active material, such as the adsorption of lithium ions to the negative electrode active material and the chemical interaction between the negative electrode active material and lithium ions. It is a concept that can also include

The negative electrode mixture layer contains a negative electrode active material as an essential component, and a conductive material, a binder, and the like as optional components. Examples of conductive materials include carbon black and carbon fibers. Examples of binders include fluorine resins, acrylic resins, rubber materials, and cellulose derivatives.

The negative electrode active material is, for example, a carbon material, and includes, for example, non-graphitizable carbon (that is, hard carbon). In non-graphitizable carbon, the Faraday reaction, in which lithium ions are reversibly absorbed and released, proceeds to develop capacity.

The non-graphitizable carbon may have an interplanar spacing of (002) planes (that is, an interplanar spacing between carbon layers) d002 of 3.8 Å or more as measured by an X-ray diffraction method. The theoretical capacity of non-graphitizable carbon is desirably 150 mAh/g or more, for example. By using non-graphitizable carbon, it becomes easier to obtain a negative electrode with a small low-temperature DCR and small expansion and contraction due to charging and discharging. The non-graphitizable carbon preferably accounts for 50 mass % or more, further 80 mass % or more, furthermore 95 mass % or more of the negative electrode active material. In addition, it is desirable that the non-graphitizable carbon accounts for 40 mass % or more, further 70 mass % or more, furthermore 90 mass % or more of the negative electrode mixture layer.

Non-graphitizable carbon and materials other than non-graphitizable carbon may be used together as the negative electrode active material. Materials other than non-graphitizable carbon that can be used as the negative electrode active material include graphitizable carbon (soft carbon), graphite (natural graphite, artificial graphite, etc.), lithium titanium oxide (spinel-type lithium titanium oxide, etc.), silicon Examples include oxides, silicon alloys, tin oxides, and tin alloys.

The average particle size of the negative electrode active material (especially non-graphitizable carbon) is preferably 1 μm or more and 20 μm or less, from the viewpoint of high filling properties of the negative electrode active material in the negative electrode and easy suppression of side reactions with the electrolyte. More preferably, the thickness is 15 μm or more. The average particle diameter is the median diameter in the volume-based particle size distribution, and can be measured, for example, by a laser diffraction particle size distribution analyzer.

The negative electrode mixture layer is formed, for example, by mixing a negative electrode active material, a conductive material, a binder, and the like with a dispersion medium to prepare a negative electrode mixture slurry, and applying the negative electrode mixture slurry to a negative electrode current collector. , formed by drying.

The negative electrode mixture layer is pre-doped with lithium ions in advance. This lowers the potential of the negative electrode, increasing the potential difference (that is, voltage) between the positive electrode and the negative electrode, thereby improving the energy density of the lithium ion capacitor. The amount of lithium to be pre-doped may be, for example, about 50% to 95% of the maximum amount that can be occluded in the negative electrode mixture layer.

The pre-doped lithium can be adhered to the surface of the negative electrode mixture layer by, for example, a vapor phase method, transfer, or the like. Vapor phase methods include methods such as chemical vapor deposition, physical vapor deposition, and sputtering.

Note that the pre-doping of lithium ions into the negative electrode mixture layer, for example, proceeds further by bringing the negative electrode mixture layer and the electrolyte into contact with each other after that, and is completed by leaving for a predetermined period of time. In addition, pre-doping of lithium ions to the negative electrode is completed by applying a predetermined charging voltage (eg, 3.4 to 4.0 V) between the terminals of the positive electrode and the negative electrode for a predetermined time (eg, 1 to 75 hours). can also

(separator)
As the separator, a cellulose fiber nonwoven fabric, a glass fiber nonwoven fabric, a polyolefin microporous film, a woven fabric or a nonwoven fabric, or the like can be used. The thickness of the separator is, for example, 8 μm or more and 300 μm or less.

FIG. 1 schematically shows the configuration of a lithium ion capacitor according to one embodiment of the present invention. The illustrated lithium ion capacitor 10 includes a wound capacitor element 1 . The capacitor element 1 is constructed by winding a sheet-like positive electrode 2 and a sheet-like negative electrode 3 with a separator 4 interposed therebetween. The positive electrode 2 and the negative electrode 3 each have a positive electrode current collector and a negative electrode current collector made of metal, and a positive electrode mixture layer and a negative electrode mixture layer supported on the surfaces thereof, respectively, and are doped and undoped with anions or lithium ions. By doing so, the capacity is expressed. For the separator 4, for example, a nonwoven fabric containing cellulose as a main component is used. A positive electrode lead wire 5a and a negative electrode lead wire 5b are connected to the positive electrode 2 and the negative electrode 3, respectively, as lead members. Capacitor element 1 is housed in a cylindrical exterior case 6 together with an electrolytic solution (not shown). The material of the exterior case 6 may be any metal such as aluminum, stainless steel, copper, iron, brass, or the like. The opening of the exterior case 6 is sealed with a sealing member 7 . The

lead wires

5 a and 5 b are led out to the outside so as to pass through the sealing member 7 . A rubber material such as butyl rubber, for example, is used for the sealing member 7 .

The maximum allowable temperature of the lithium ion capacitor according to the present disclosure is, for example, 80°C or higher, and may be 85°C or higher. The maximum permissible temperature is the maximum ambient temperature at which the capacitor can be used continuously. The maximum allowable temperature is, for example, the maximum ambient temperature at which the lithium ion capacitor can be used guaranteed by the manufacturer of the lithium ion capacitor to the purchaser, and is described in catalogs, pamphlets, product specifications, and the like. The maximum permissible temperature may be a numerical value calculated from a relational expression between the nominal temperature coefficient, the capacitance at 25° C., and the capacitance at the maximum permissible temperature.

The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to the examples.

(1) Positive Electrode Current Collector A plurality of kinds of aluminum foils with different capacitances were prepared. The capacitance of the aluminum foil was controlled by changing the surface roughness of the aluminum foil. The thickness of the aluminum foil with the smallest capacitance was 20 μm, and the surface roughness of the aluminum foil was changed by etching this aluminum foil.

(2) Electrolyte _LiPF6 was used as the first lithium salt, and LiFSI was used as the second lithium salt. A mixture of propylene carbonate and dimethyl carbonate at a volume ratio of 1:1 was used as a solvent. 0.2% by mass of vinylene carbonate was included in the solvent. An electrolyte was prepared by dissolving a predetermined lithium salt in a solvent at a predetermined concentration. The total molar concentration of the first lithium salt (LiPF ₆ ) and the second lithium salt (LiFSI) in the electrolyte was fixed at 1.2 mol/L. A plurality of types of electrolytes having different ratios of the molar concentration of the first lithium salt (LiPF ₆ ) to the total molar concentration were prepared.

(3) Preparation of positive electrode 88 parts by mass of activated carbon (average particle size 5.5 μm) as a positive electrode active material, 2 parts by mass of polytetrafluoroethylene as a binder, and 4 parts by mass of carboxycellulose as a thickener , and 6 parts by mass of acetylene black, which is a conductive material, were dispersed in water to prepare a positive electrode mixture slurry. The resulting positive electrode mixture slurry was applied to both surfaces of a predetermined aluminum foil, the coating film was dried and rolled to form a positive electrode mixture layer, and a positive electrode was obtained. A positive electrode lead wire was connected to the aluminum foil as a lead member.

(4) Preparation of Negative Electrode 97 parts by mass of non-graphitizable carbon (average particle size 5 μm), 2 parts by mass of styrene-butadiene rubber as a binder, 1 part by mass of carboxycellulose as a thickener, and a conductive material. 6 parts by mass of Ketjenblack was dispersed in water to prepare a negative electrode mixture slurry. The resulting negative electrode mixture slurry was applied to both sides of a copper foil having a thickness of 10 μm, and the coating film was dried and rolled to form a negative electrode mixture layer, thereby obtaining a negative electrode. A negative electrode lead wire was connected to the copper foil as a lead member.

After that, a thin film of metallic lithium for pre-doping was formed on the entire surface of the negative electrode mixture layer by vacuum deposition. The amount of lithium to be pre-doped was set so that the negative electrode potential in the electrolyte after pre-doping was completed was 0.2 V or less with respect to metallic lithium.

(5) Fabrication of Capacitor Element A capacitor element was formed by winding a negative electrode and a predetermined positive electrode in a columnar shape with a cellulose nonwoven fabric separator (thickness: 25 μm) interposed therebetween. At this time, each lead wire was made to protrude from one end surface of the wound body.

(6) Assembly of Lithium Ion Capacitor A capacitor element is housed in a bottomed cell case having an opening, and after a predetermined electrolyte is injected into the cell case, each lead wire passes through the sealing member and leads to the outside. Thus, the opening of the cell case was closed with the sealing member, and the lithium ion capacitor as shown in FIG. 1 was assembled.

After that, aging was performed at 60°C while applying a charging voltage of 3.8 V between the terminals of the positive electrode and the negative electrode to complete the pre-doping of lithium ions to the negative electrode.

(7) Evaluation (measurement of DCR)
The lithium ion capacitor immediately after aging was subjected to constant current charging at a current density of 2 mA/cm ² per positive electrode area until the voltage reached 3.8 V in an environment of −30° C., and then to a voltage of 3.8 V. was maintained for 10 minutes. After that, in an environment of −30° C., constant current discharge was performed at a current density of 2 mA/cm ² per positive electrode area until the voltage reached 2.2V.

Next, using the discharge curve (vertical axis: discharge voltage, horizontal axis: discharge time) obtained in the above discharge, the first-order An approximate straight line was obtained, and the voltage VS of the intercept of the approximate straight line was obtained. A value (V0−VS) obtained by subtracting the voltage VS from the voltage V0 at the start of discharge (0 seconds after the start of discharge) was obtained as ΔV. Using ΔV (V) and the current value during discharge (current density per positive electrode area 2 mA/cm ² ×positive electrode area), the internal resistance (DCR) (Ω) was obtained from the following formula. Id in the formula is the current value during discharge (current density per positive electrode area: 2 mA/cm ² ×positive electrode area).

The positive electrode current collector has a capacitance of 4 μF/cm ² , and the first lithium salt (LiPF ₆ ) accounts for the total molar concentration of the first lithium salt (LiPF ₆ ) and the second lithium salt (LiFSI). Relative values are shown in Table 1, assuming that R1 is 100 when the molar concentration ratio is 0% (that is, when all the lithium salts are LiFSI). The smaller the number, the lower the DCR.

　DCR = ΔV/Id

(Float test)
A constant voltage of 3.8 V was applied to the lithium ion capacitor under an environment of 85° C., and float charging was performed for 1000 hours, and the amount of gas generated during the float charging was determined. The amount of gas generated during float charging was calculated from the swelling amount of the rubber sealing member. The positive electrode current collector has a capacitance of 4 μF/cm ² , and the first lithium salt (LiPF ₆ ) accounts for the total molar concentration of the first lithium salt (LiPF ₆ ) and the second lithium salt (LiFSI). Relative values are shown in Table 2, assuming that the amount of gas generated is 100 when the molar concentration ratio is 0% (that is, when the lithium salt is all LiFSI). It can be said that the smaller the value, the smaller the amount of gas generated, the less the side reaction between the positive electrode current collector and the electrolyte, and the less likely the positive electrode will deteriorate.

From Tables 1 and 2, the capacitance of the positive electrode current collector is 20 μF/cm ² or less, and the ratio of the molar concentration of the first lithium salt to the total molar concentration of the lithium salts in the electrolyte is greater than 0%, It can be understood that when the ratio is 35% or less, well-balanced results can be obtained in which deterioration of the positive electrode can be suppressed while suppressing DCR at low temperatures.

Next, the ratio of the molar concentration of the first lithium salt (LiPF ₆ ) to the total molar concentration of the first lithium salt (LiPF ₆ ) and the second lithium salt (LiFSI) in the electrolyte is fixed at 10%, and the total A plurality of types of electrolytes with different molar concentrations were prepared. Lithium ion capacitors were assembled and aged in the same manner as described above, except that the prescribed electrolyte was used, to complete the pre-doping of lithium ions into the negative electrode.

Fig. 2 shows the relationship between the amount of gas generated during DCR at -30°C and float charge at 85°C of a lithium ion capacitor and the total molar concentration of lithium salts (salt concentration M). From FIG. 2, it can be understood that the total molar concentration of lithium salts in the electrolyte is preferably 0.7 mol/L or more and 1.3 mol/L or less, or 0.7 mol/L or more and 1.0 mol/L or less.

The lithium ion capacitor according to the present invention is suitable for applications with a maximum allowable temperature of 80°C or higher, or 85°C or higher, and is suitable for in-vehicle use, for example.

1: capacitor element, 2: positive electrode, 3: negative electrode, 4: separator, 5a: positive electrode lead wire, 5b: negative electrode lead wire, 6: exterior case, 7: sealing member, 10: lithium ion capacitor

Claims

comprising a positive electrode, a negative electrode and a lithium ion conductive electrolyte;
The positive electrode includes a positive electrode current collector and a positive electrode mixture layer supported on the positive electrode current collector,
The positive electrode mixture layer includes a positive electrode active material that reversibly dopes anions,
The positive electrode current collector has a capacitance of 20 μF/cm 2 or less,
The negative electrode includes a negative electrode current collector and a negative electrode mixture layer supported on the negative electrode current collector,
The negative electrode mixture layer includes a negative electrode active material reversibly doped with lithium ions,
the electrolyte comprises a first lithium salt and a second lithium salt;
The first lithium salt is a lithium salt of a fluorine-containing inorganic acid,
The second lithium salt is a lithium salt of a fluorine-containing acid imide,
A lithium ion capacitor, wherein the ratio of the molar concentration of the first lithium salt to the total molar concentration of the first lithium salt and the second lithium salt in the electrolyte is more than 0% and 35% or less.
When the molar concentration ratio of the first lithium salt is X% and the capacitance of the positive electrode current collector is Y μF/cm 2 , X and Y are
Formula (1): Y≤X+10
Formula (2): Y≤-0.8X+28
The lithium ion capacitor according to claim 1, which satisfies:
3. The lithium ion capacitor according to claim 1, wherein the positive electrode current collector has a capacitance of 4 μF/cm 2 or less.
The total molar concentration of the first lithium salt and the second lithium salt in the electrolyte is 0.7 mol/L or more and 1.3 mol/L or less, according to any one of claims 1 to 3. of lithium-ion capacitors.
The lithium ion capacitor according to any one of claims 1 to 4, wherein the positive electrode active material contains activated carbon.
The first lithium salt is at least one selected from the group consisting of LiPF 6 , LiBF 4 , LiSbF 6 and LiAsF 6 ,
The second lithium salt is LiN ( FSO2 ) 2 , LiN ( CF3SO2 ) 2 , LiN ( CF3SO2 ) ( C4F9SO2 ) and LiN ( C2F5SO2 ) 2 The lithium ion capacitor according to any one of claims 1 to 5, which is at least one selected from the group consisting of:
The lithium ion capacitor according to any one of claims 1 to 6, wherein the maximum allowable temperature is 80°C or higher.
The lithium ion capacitor according to any one of claims 1 to 7, wherein the maximum allowable temperature is 85°C or higher.